\begin{tikzpicture}[>=Stealth]
    \draw [->] (-1,0) -- (4.8,0) node[anchor=north] {$n$};
    \draw [->] (0,-1) -- (0,8.5) node[anchor=east] {$a_n$};
    \node at (-0.3,-0.3) {$O$};
    \foreach \x in {1,...,4} {
        \draw (\x,0.2) -- (\x,0) node[anchor=north] {$\x$};
    }
    \foreach \y in {1,...,8} {
        \draw (0.2,\y) -- (0,\y) node[anchor=east] {$\y$};
    }

    \draw[domain=-0.2:4.1,smooth,samples=50] plot (\x, {0.5 * 2^\x});
\end{tikzpicture}